1. Field of the Invention
The present invention relates to an electrophotographic printer.
2. Description of the Related Art
FIG. 35 illustrates a general construction of a conventional electrophotographic printer. FIG. 36 is a side view of a paper transporting path of the conventional electrophotographic printer. Referring to FIG. 35, a photoconductive drum 11 rotates in a direction shown by arrow A about a rotational axis passing through a center O. A charging roller 12 receives a negative voltage and rotates in contact with the photoconductive drum 11, so that the entire surface of the photoconductive drum 11 is negatively uniformly charged. The charged surface of the photoconductive drum 11 is then exposed to image light emitted from a LED head 13, so that an electrostatic latent image is formed on the surface. Then, the electrostatic latent image is transported to a developing section 14 where the electrostatic latent image is developed with charged toner 15 into a toner image.
Then, print paper 16 is fed to a transfer point P1 where the photoconductive drum 11 is in contact with a transfer roller 17 that rotates in a direction shown by arrow B. When the paper 16 reaches the transfer point P1, a transfer power supply 18 applies a positive voltage to the transfer roller 17 so as to develop an electric field across the photoconductive drum 11 and the transfer roller 17. The electric field exerts a Coulomb force on the toner 15 so that the toner 15 is attracted to the print paper 16. In this manner, the toner image is transferred from the photoconductive drum 11 to the print paper 16. Some of the toner 15 still remains on the surface of the photoconductive drum 11 after the toner image has been transferred to the print paper 16. Such residual toner is recovered by a cleaning device 19.
With the aforementioned conventional electrophotographic printers, the transfer voltage needs to be maintained within a range of transfer voltage in which a good transfer operation can be carried out, in order to achieve good printing result without deterioration of image quality of the print.
A voltage lower than a lower limit of the optimum transfer-voltage range causes blurred images. In contrast, a voltage higher than an upper limit of the range causes too high an electric field with the result that the toner particles are forced to be pulled from the surface of the photoconductive drum toward the transfer roller before the toner particles are normally carried by the photoconductive drum to the transfer point P1. In any case, the transfer result is poor. When the toner particles are pulled away from the photoconductive drum, the image quality is seriously deteriorated.
One solution is to control the transfer power supply 18 so that the transfer voltage is within the optimum transfer-voltage range in which reasonable transfer can be accomplished. However, good transfer result can be obtained only in a narrow range of transfer voltage and therefore it is difficult to control the transfer voltage within such a narrow range. In addition, good transfer result can be obtained in different ranges of transfer voltage depending on the kind of print paper 16 and environmental conditions (temperature, humidity, and so on) in which the electrophotographic printer is placed. Thus, the transfer voltage needs to be changed in accordance with, for example, the kind of print paper 16 and the environmental conditions. This makes the control of transfer voltage more difficult.